It is only in the last few decades that substantial research efforts have been focused on the interactions of river discharge with tidal waves and storm surges into regions beyond the limit of salinity intrusion. A recent article in Reviews of Geophysics, Tidal river dynamics: Implications for deltas, provides a description of state-of-the-art methods for a comprehensive analysis of water levels, wave propagation, discharges, and inundation extent in tidal rivers is provided. Implications for lowland river deltas are also discussed in terms of sedimentary deposits, channel bifurcation, avulsion, and salinity intrusion, addressing contemporary research challenges. AGU asked the authors of the article to highlight the important results that have emerged from their research and some of the important questions that remain.

What is a tidal river?

A tidal river is the part of a river-estuary system where there are strong interactions between tides and river flow. River processes increasingly dominate over tidal processes farther upstream. In large rivers like the Yangtze and Amazon, these interactions may extend many hundreds of kilometers upstream past the upstream limit of salinity intrusion. In the more seaward parts of a tidal river, the average tidal range and the seasonal variations in river stage are often about equal. Farther upstream, tidal range decreases during periods of high river flow, and seasonal river stage variations greatly exceed tidal amplitudes. Defining boundaries for tidal rivers has proven complicated and controversial, in part because administrative definitions may not map well onto the actual dynamics. It seems logical that the tidal river should begin at the landward boundary of an estuary – but where is that? Estuaries have been defined by some scientists to extend only to the landward limit that saline water intrudes into the river. But this limit varies strongly, and the Amazon has no salinity intrusion at all, yet it certainly has an estuary. Others have suggested that an estuary extends upstream to the tidal limit. This can be 500 to almost 1000 km from the ocean in some large rivers; such a definition is counterintuitive. We thus defined the seaward boundary of the tidal river to occur at the point where monthly variations in stage exceed monthly variations in tidal range. In the tidal river upstream of this point, the lowest tidal water levels will occur during neap tides not spring tides, as would be the case near the ocean. Usually, this boundary is seasonally stable; whereas a definition based on the salinity intrusion limit is not. In both definitions, the landward tidal-river boundary coincides with the reasonably stable upstream limit of tidal intrusion.

Why is this topic timely and important?

Tidal rivers are hotspots of climate change impact. They are affected both by changing river discharge regimes and by rising sea levels. Knowledge about the processes governing tidal river dynamics has implications for the way climate change impacts are perceived. For example, sea level rise may not directly translate into a rise of extreme water levels because of corresponding changes in the dynamics of the tidal river. In channels across the freshwater part of the Rhine-Meuse delta, for example, mean water levels have increased at the same pace as the mean sea level, but both the high and the low extremes have dropped. The reduction of the lowermost water levels in summer now causes enhanced salinity intrusion, jeopardizing freshwater availability in southwestern parts of The Netherlands. Over the length of a tidal river, water levels govern the inundation frequency of the adjacent wetlands. Ecosystems may have a capacity to adapt to a changing inundation regime, but in the near future, this adaptation capacity will be put to the test by discharge regime changes in response to climate change, and human mitigation measures. The importance of tidal river dynamics is further manifest in delta geology and management. In natural deltas without engineered dikes, tidal motions often largely control the chance of a levee breach, and cause rhythmic patterns of sediment deposition. In human controlled systems such as the Mississippi Delta, river-flows may control flood frequency, but it is questionable whether tidal effects can be ignored when developing flood control measures.

What recent advances in particular are leading to a new understanding or synthesis?

The study of tidal river dynamics unites two disciplines, hydrology and physical oceanography, in ways that are perhaps unexpected in either field. Hydrologists have long ignored the tides. In studies of estuaries, physical oceanographers often consider river discharge as either constant or absent, and tides as stationary. Improved methods of time-series analysis and multiscale numerical modeling facilitate studies of tidal rivers that combine the two worlds. A major challenge in tidal river water level dynamics is to identify how individual tidal waves, which can readily be predicted in coastal waters, develop as they propagate upstream in a shallow river and interact with the discharge, and with tides of other frequencies. The harmonic analysis method, generally used for water level prediction in harbors, assumes that the tidal motion is statistically stationary, an assumption that is invalid in a tidal river. Wavelet analysis is appropriate for nonstationary data series, but has low frequency resolution. Recent techniques have improved harmonic analysis, to address nonstationary processes. Results have shown, for example, that a simple relation often exists between river discharge and the ratio of amplitudes in a tidal river and at the coast. Flexible mesh numerical flow models have further contributed to improved understanding of the land-sea continuum. Contemporary model simulation environments allow a holistic, integrated approach, where the effects of the tidal motion and sea-level variation can be studied in a tidal river and its adjacent wetlands.

What are the implications for society (e.g., cities, coastal engineering, flood control, water supply, shipping, fisheries, and recreation) of this new synthesis and understanding of tidal rivers. That is, can you provide some brief examples of their importance and how the new understandings help?

Recent understandings support the need to abandon the use of harmonic analysis for prediction of river tides, for example by harbour authorities. Models and new tools for analysis of tidal river dynamics can be used to better predict water levels, an objective that is directly relevant to navigation, water supply, habitat restoration, and fisheries management. Simple models are now available that can be used to explore how engineering activity, such as channel deepening by dredging, may affect the highest and lowest water levels in a tidal river. Higher river discharges attenuate the tide, so that an increased discharge does not necessarily result in an increased flood risk in the lower reaches of a tidal river. In the landward reaches of a tidal river, high river discharges damp tidal motions, such that changes in discharge have a more direct consequence for water level dynamics, and possible flooding. Understanding tidal rivers is also key to controlling salinity intrusion, relevant for drinking water supply and agriculture. In particular, tidal rivers modulate river discharge, introducing fortnightly variations that are predictable, and thus can be anticipated. It is likely that new understanding of tidal river dynamics and novel time-series analysis methods can be used to develop simple tools to predict salinity intrusion length. In a similar vein, the latest insights and tools can support river restoration efforts, offering support to the management of water level regimes, which are linked to ecosystem zonation.

What are the major unsolved or unresolved questions, and where are additional data or modeling efforts needed?

The complexity of tides in freshwater environments increases in branching channels networks. Potentially, the tidal motion can become chaotic. More research is needed to explore tidal propagation, sediment transport, and geomorphology in such systems. Much can also be learned from the interpretation of existing long-term water level records. Records for many river harbors extend at least a century, providing a wealth of information. In conjunction with hindcasts of the river discharge and ocean tidal amplitudes, historical analyses of harbor tides can reveal natural and anthropogenic trends in tidal river dynamics, and possible regime changes. There is also a lack of knowledge about the subaqueous geomorphology of tidal rivers, which cause spatial and temporal variations in the degree in which the bed resists the flow. In particular, the development of dunes in tidal rivers where the sediment characteristics transition from sand to mud needs to be better understood. New techniques for in situ monitoring can assist further exploration of the associated processes governing flow and sediment transport. Finally, there is a gap in knowledge of the interactions between tidal river hydrodynamics and wetland vegetation. Holistic modelling efforts are needed to better identify the tolerance of wetland species to shifts in water level regimes.

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